Fuel efficient membrane electrode assembly

ABSTRACT

The present invention provides a fuel efficient membrane electrode assembly that substantially resists methanol from “crossing over” by sealing the anode diffusion layer and catalyst layer, and by preventing fluids from passing across at least a portion of the membrane electrolyte. The efficiency of the fuel cell is maintained because essentially all of the fuel is reacted on the catalyst layers creating electricity, which is transferred to the fuel cell load. The temperature of the fuel cell is also maintained as the uncatalyzed reaction that produces heat is prevented.

RELATED APPLICATIONS

This application is a divisional of U.S. Ser. No. 10/449,271, filed byRobert S. Hirsch on May 30, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to direct oxidation fuel cells, andmore particularly, to components for managing fluids within such fuelcells.

2. Background Information

Fuel cells are devices in which electrochemical reactions are used togenerate electricity. A variety of materials may be suited for use as afuel depending upon the nature of the fuel cell. Organic materials, suchas methanol or natural gas, are attractive fuel choices due to the theirhigh specific energy.

Fuel cell systems may be divided into “reformer-based” systems (i.e.,those in which the fuel is processed in some fashion to extract hydrogenfrom the fuel before it is introduced into the fuel cell) or “directoxidation” systems in which the fuel is fed directly into the cellwithout the need for separate internal or external processing. Mostcurrently available fuel cells are reformer-based fuel cell systems.However, because fuel processing is complex, and requires expensivecomponents, which occupy comparatively significant volume, the use ofreformer based systems is presently limited to comparatively large, highpower applications.

Direct oxidation fuel cell systems may be better suited for a number ofapplications in smaller mobile devices (e.g., mobile phones, handheldand laptop computers), as well as in some larger scale applications. Indirect oxidation fuel cells of interest here, a carbonaceous liquid fuelin an aqueous solution (typically aqueous methanol) is applied to theanode face of a membrane electrode assembly (MEA). The MEA contains aprotonically conductive, but electronically non-conductive membrane(PCM). Typically, a catalyst, which enables direct oxidation of the fuelon the anode aspect of the PCM, is disposed on the surface of the PCM(or is otherwise present in the anode chamber of the fuel cell). In thefuel oxidation process at the anode, the products are protons, electronsand carbon dioxide. Protons (from hydrogen in the fuel and watermolecules involved in the anodic reaction) are separated from theelectrons. The protons migrate through the PCM, which is substantiallyimpermeable to the electrons. The electrons travel through an externalcircuit, which includes the load, and are united with the protons andoxygen molecules in the cathodic reaction, thus providing electricalpower from the fuel cell.

One example of a direct oxidation fuel cell system is a direct methanolfuel cell system or DMFC system. In a DMFC system, a mixture comprisedpredominantly of methanol and water is used as fuel (the “fuelmixture”), and oxygen, preferably from ambient air, is used as theoxidizing agent. The fundamental reactions are the anodic oxidation ofthe methanol and water in the fuel mixture into CO₂, protons, andelectrons; and the cathodic combination of protons, electrons and oxygeninto water. The overall reaction may be limited by the failure of eitherof these reactions to proceed at an acceptable rate (more specifically,slow oxidation of the fuel mixture will limit the cathodic generation ofwater, and vice versa).

Direct methanol fuel cells are being developed towards commercialproduction for use in portable electronic devices. Thus, the DMFCsystem, including the fuel cell and the other components should befabricated using materials and processes that not only optimize theelectricity-generating reactions, but which are also cost effective.Furthermore, the manufacturing process associated with a given systemshould not be prohibitive in terms of associated labor or manufacturingcost or difficulty.

Typical DMFC systems include a fuel source, fluid and effluentmanagement and air management systems, and a direct methanol fuel cell(“fuel cell”). The fuel cell typically consists of a housing, hardwarefor current collection and fuel and air distribution, and a membraneelectrode assembly (“MEA”) disposed within the housing.

A typical MEA includes a centrally disposed, protonically conductive,electronically non-conductive membrane (“PCM”). One example of acommercially available PCM is NAFION® a registered trademark of E.I.Dupont de Nemours and Company, a cation exchange membrane comprised ofpolyperflourosulfonic acid, in a variety of thicknesses and equivalentweights. The PCM is typically coated on each face with anelectrocatalyst such as platinum, or platinum/ruthenium mixtures oralloy particles. On either face of the catalyst coated PCM, theelectrode assembly typically includes a diffusion layer. The diffusionlayer on the anode side is employed to evenly distribute the liquid fuelmixture across the anode face of the PCM, while allowing the gaseousproduct of the reaction, typically carbon dioxide, to move away from theanode face of the PCM. In the case of the cathode side, a diffusionlayer is used to achieve a fast supply and even distribution of gaseousoxygen across the cathode face of the PCM, while minimizing oreliminating the collection of liquid, typically water, on the cathodeaspect of the PCM. Each of the anode and cathode diffusion layers alsoassist in the collection and conduction of electric current from thecatalyzed PCM.

The diffusion layers are conventionally fabricated of carbon paper or acarbon cloth, typically with a thin, porous coating made of a mixture ofcarbon powder and TEFLON®. Such carbon paper or carbon cloth componentsallow a relatively high flux of methanol when immersed in a liquidmethanol and water fuel mixture. While some methanol access through theanode diffusion layer is required for maintaining anode, and therefore,cell current, a high flux of methanol through the anode diffusion layeris a shortcoming because most presently available membrane electrolytessuitable for use in a DFMC system are typically permeable to methanoland concentrated fuel which, if introduced into the anode chamber, canthus pass at a significant rate through the diffusion layer and themembrane and oxidize on the cathode face of the membrane. This resultsin wasted fuel as well as diminished cathode performance, leading todiminished performance of the fuel cell and fuel cell system.

Traditional DMFC structures have required that the diffusion layersperform a current conduction function as well as managing theintroduction and removal of reactants and products within the MEA. Thus,these layers have had to be electrically conductive, as well as capableof managing the transport of liquids and gasses within the MEA, i.e.,transport reactants to and products away from the catalyst coated PCM.Diffusion layers used in fuel cells are comprised of porous carbon paperor carbon cloth, typically between 100-500 microns thick. Each of thesediffusion layers is typically “wet-proofed” with TEFLON® or otherwisetreated in a manner that makes the diffusion layer hydrophobic toprevent liquid water from saturating the diffusion layer. Such“wet-proofing” may not be ideal for the anode of a DMFC or other directoxidation fuel cell system.

A metallic diffusion layer or a metallic diffusion layer combined with aflow field plate in a direct oxidation fuel cell has been described foruse as a controlled methanol transport barrier. The metallic layercomponent can be manufactured using particle diffusion bondingtechniques as described in commonly owned U.S. patent application Ser.No. 09/882,699 which was filed on Jun. 15, 2001, for a METALLIC LAYERCOMPONENT FOR USE IN A DIRECT OXIDATION FUEL CELL.

Those skilled in the art will recognize that materials other than metalsmay offer advantages for certain architectures or designs. For example,many polymers are less expensive, and easier to mold or form into adesired structure than metals, provided that there are alternatestructures and methods in place to collect current and provide otherdesired characteristics. In addition, the use of polymers allow forprecise engineering of the size and shape of the pores in the component,and may be further desirable as it is possible to utilize a liquidimpermeable polymer.

As noted, the MEA is formed of a PCM to which a catalyst is applied,forming a catalyst coated membrane (CCM). Diffusion layers are pressedonto the CCM. Generally, the entire MEA is held in place by a framecomprised of a faceplate that is disposed on each of the anode side andthe cathode side. The faceplates provide an electron path while alsoproviding compression to the MEA. The faceplates also physically connectthe MEA to the fuel cell system. It is common to make the diffusionlayers and catalyst layers the same dimensions as the openings of theframe. Fluid leaks are resisted by gaskets that are placed between thefaceplates, around the MEA. However, this is not always successful, asthere is an another path, around the diffusion layers, and across thecatalyst layer, through which fuel may pass or leak from the anode tothe cathode side of the PCM. This fuel crosses over the membrane,resulting in methanol cross over. The fuel is thus wasted as it does notcontribute to the electricity generating reactions. Instead, it oxidizeson the cathode aspect of the MEA, thus creating heat. Excess heat canresult in changes in the behavior of the NAFION® membrane, thus reducingfuel cell efficiency. Alternatively, the fuel can simply remain on thecathode aspect of the PCM also reducing fuel cell efficiency.

There remains a need, therefore, for an improved membrane electrodeassembly in which undesirable leaks across or around the diffusionlayers and/or the catalyst or other components are eliminated, withoutadding unnecessary bulk and weight to the fuel cell, and fuel cellsystem.

It is thus an object of the present invention to enhance fuel cellefficiency by eliminating possible paths for fluid to leak from theanode to the cathode of an MEA.

SUMMARY OF THE INVENTION

The disadvantages of prior techniques are overcome by the solutionsprovided by the present invention, which include techniques forincreasing fuel efficiency in a direct oxidation fuel cell by preventingfuel leakage around the anode diffusion layer and/or the catalystcoating on the anode face of the membrane electrolyte. The techniques ofthe present invention substantially resist fuel from flowing inundesired paths from or around the anode to the cathode side of the MEAwithout generating electricity, thereby maintaining the efficiency ofthe fuel cell. This may be accomplished in accordance with a number ofdifferent embodiments of the invention described in further detailherein but which either render a portion of the membrane electrolytesubstantially impermeable to methanol and water, and/or physicallyprevent fuel from leaking around the diffusion layers and othercomponents.

For purposes of clarity of illustration, we have herein described anembodiment of the invention having a fuel cell that includes a diffusionlayer on the cathode side of the cell and a diffusion layer on the anodeside. The fuel cell, however, may also include combinations of otherlayers, components, and/or membranes, which elements may control ormanage the substances in the cell (fuel, water, carbon dioxide andoxygen) in a number of different variations. It is expresslycontemplated that one or more of such elements may also, oralternatively, be included in the sealing techniques of the presentinvention (such as, for example, by being extended under the gasketing)to prevent undesired leakage, in accordance with the invention and whileremaining within the scope of the present invention.

In accordance with a first aspect of the invention the chemicalcomposition of the a portion of the PCM, such as a peripheral area ofthe PCM, beyond the catalyst coating, is rendered substantiallyimpermeable to fuel, and/or ionically non-conductive. This means thatneither fuel nor water will be able to pass through the impermeableareas of the PCM, forcing fuel to flow through the catalytic coating andresulting in electricity-generating reactions, and thus increasing fuelefficiency.

In accordance another aspect of the invention, the diffusion layers areextended beyond the impermeable area of the PCM, forming a seal. Thissubstantially prevents the fuel solution from leaking between the edgesof the diffusion layers and the membrane electrolyte. Even though theanode diffusion layer allows the fuel solution to be introduced to theimpermeable area of the PCM it acts as a physical barrier, thus reducingthe amount of fuel solution to only that which diffuses across the anodediffusion layer.

In accordance with another aspect of the invention, the catalytic layersare extended. This also substantially resists the flow of fuel substancethrough the aforementioned path by providing catalytic area for fuelpassing through the anode diffusion layer and consequently the methanolor other fuel substance is oxidized on the catalyst. It also provides acatalytic surface for fuel that may have escaped the anode diffusionlayer. In most cases, the first and second aspects will be combined inthe fuel cell system.

Further, in accordance with yet another aspect of the invention, a fuelimpermeable material, such as TEFLON®, is painted or otherwise appliedto a portion of the PCM such as the outer edges forming a border aroundthe PCM in such a fashion that it overlays the edge of the activecatalyst area. Alternatively, a plastic gasketing can be used to createthe seal. The edges of the associated anode diffusion layer typicallyextend at least to the TEFLON® or the gasketing and this creates a sealbetween the catalyst and the PCM. In addition, it may be desirable toapply such a fuel impermeable material in such a fashion that it “seals”the diffusion layer and catalyst layer to the PCM so that fuel does notescape from the diffusion layer to the PCM.

The aspects of the present invention can be combined in various wayswith other aspects of the present invention in a fuel cell system, whileremaining within the scope of the present invention. In addition, otherdesigns and architectures, in which it is desirable to manage the flowof fuel to a protonically conductive membrane electrolyte are within thescope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention description below refers to the accompanying drawings, ofwhich:

FIG. 1 is a schematic diagram of a DMFC system in which the presentinvention can be employed;

FIG. 2 is a cross-sectional diagram of a prior art device illustratingpossible leakage paths;

FIG. 3 is a schematic cross section of one embodiment of the inventionin which a peripheral area of the PCM is chemically altered;

FIG. 4 is a schematic top plan view of a section of the component ofFIG. 3;

FIG. 5 is a schematic cross section of one embodiment of the inventionin which a physical sealant is used in area of the catalyzed layer;

FIG. 6 is a schematic top plan view of a section of the component ofFIG. 5;

FIG. 7 is a schematic cross-section of another embodiment in which anumber of the techniques of the present invention are combined; and

FIG. 8 is a cross-sectional diagram of another embodiment of the presentinvention with gasketing extending over the diffusion layers.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

An example of a direct oxidation fuel system 20 is schematicallyillustrated in FIG. 1. The fuel cell system 20 includes a directoxidation fuel cell, which may be a direct methanol fuel cell 21(“DMFC”), for example. For purposes of illustration, and not by way oflimitation, we herein describe an illustrative embodiment of theinvention with DMFC 21, and a DMFC system with the fuel substance beingmethanol or an aqueous methanol solution. However, it is within thescope of the present invention that other carbonaceous fuels such asethanol, or combinations of carbonaceous fuels and aqueous solutionsthereof may be used. It should be further understood that the inventionis applicable to any fuel cell system where it is preferable tointroduce a liquid fuel, vaporous fuel or component thereof to the anodeaspect 26 of a membrane electrode assembly (MEA) 24 in the mannerdescribed herein, and is not limited to the embodiment described inFIG. 1. It should thus be understood that while the invention may beimplemented in the system illustrated in FIG. 1, it is equallyapplicable and can be readily employed in other architectures thatinclude a DMFC that has a protonically conductive, electronicallynon-conductive membrane as well DMFC systems and/or direct oxidationfuel cell stacks.

The system 20, including the DMFC 21, has a fuel delivery system todeliver fuel from fuel source 22, which may be a fuel reservoir or fuelcartridge. The DMFC 21 includes a housing 23 that encloses an MEA 24.MEA 24 incorporates protonically conductive, electronicallynon-conductive, membrane (PCM) 25, and typically includes at least onediffusion layer in contact with one or both aspects of the PCM 25. PCM25 has an anode face 26 and cathode face 27, each of which may be coatedwith a catalyst, including but not limited to platinum, or a blend ofplatinum and ruthenium, or combinations thereof and alloys thereof.Diffusion layers are usually fabricated from carbon cloth or carbonpaper that are treated with a mixture of TEFLON® and high surface areacarbon particles, are typically provided in intimate contact with thecatalyzed faces of each of the anode 26 and cathode 27 aspects of thePCM 25, though the invention is not limited to systems that requirediffusion layers. The portion of DMFC 21 defined by the housing 23 andthe anode face 26 of the PCM 25 is referred to herein as the anodechamber 28. The portion of DMFC 21 defined by the housing 23 and thecathode face 27 of the PCM 25 is referred to herein as the cathodechamber 29. The anode chamber 28 and cathode chamber 29 may furthercontain a flow field plate or plates (not shown) in contact with thediffusion layer, in order to manage the mass transport of reactants andproducts of the reaction. The anode chamber 28 may also include amethanol delivery film which may be a pervaporation membrane thatchanges a liquid fuel to a vaporous fuel, as is described incommonly-owned U.S. patent application No. 10/413,983 for a DIRECTOXIDATION FUEL CELL OPERATING WITH DIRECT FEED OF CONCENTRATED FUELUNDER PASSIVE WATER MANAGEMENT, filed on Apr. 15, 2003, which ispresently incorporated herein by reference. Water management componentsmay also be included in either the anode chamber or the cathode chamber,as described in the cited application.

Those skilled in the art will recognize that the catalyst may be appliedto the PCM 25 by applying a suspension containing the catalyst to PCM25. As used herein the terms “anode face” and “cathode face” may referto the catalyzed faces of the PCM 25, and shall include any residualcatalyst materials that may remain on the surface of the PCM 25 as theresult of such application.

As will be understood by those skilled in the art,electricity-generating reactions occur when a carbonaceous fuel mixture,including, but not limited to methanol or an aqueous methanol solutionis introduced to the anode face 26 of the MEA, and oxygen, usually fromambient air, is introduced to the cathode face 27 of the MEA. The fuelmixture passes through channels in the flow field plate (or is presentin the anode chamber 28), and/or a diffusion layer, and is ultimatelypresented to the anode face 26 of the PCM 25. Catalysts on the membranesurface (or which are otherwise present within the MEA 24) enable theanodic oxidation of the carbonaceous fuel on the anode face 26,separating hydrogen protons and electrons from the fuel and watermolecules of the fuel mixture. Upon the closing of a circuit, protonspass through PCM 25, which is impermeable to the electrons. Theelectrons thus seek a different path to reunite with the protons, andtravel through a load 31 of an external circuit, thus providingelectrical power to the load 31. So long as the reactions continue, acurrent is maintained through the external circuit. The presentinvention describes techniques for preventing leakage through undesiredpaths in the DMFC as described herein. Direct oxidation fuel cellstypically produce water (H₂O) and carbon dioxide (CO₂) as products ofthe reaction, which must be directed away from the catalyzed anode andcathode membrane surfaces 26, 27. The gas separator 32 separates theexcess air and water vapor from the water. This water can be laterdirected to the pump 30 via a flow path 33. Those skilled in the artwill recognize that the gas separator 32 may be incorporated into anexisting component within the DMFC 21 or the DMFC system 20.

FIG. 2 illustrates a prior art fuel cell portion 24 comprising a PCM 25that is contained within an anode faceplate 41, (which may be part of acompression frame). The faceplate 41 has an opening (not shown) throughwhich fuel is introduced to an MEA 26. A cathodic faceplate 42 allowsoxygen from a suitable source to enter the cathode portion of the MEA26. Beneath the faceplates 41, 42 are an anode diffusion layer 43, whichis in contact with a catalyst coating 45 on the anode face of the PCM25. The cathode diffusion layer 44 is in contact with the catalystcoating 46 on the cathode face of the PCM 25. It is noted that in thisconventional design, the diffusion layers, and the catalyst layers arethe same lengths. For example, anode diffusion layer 43 is the samelength as the catalyst layer 45. Because of this arrangement of thediffusion layers 43 and 44, and the catalyst coatings 45 and 46, thereare gaps between the MEA 26 and the faceplates 41 and 42. Typically,these gaps are sealed by gaskets 47 and 48. However, even withgasketing, smaller gaps 51, 52 on the anode side and gaps 53, 54 on thecathode side still remain. These gaps allow fuel to leak through to thecathode side, where it is wasted without generating electricity.

The present invention provides solutions to these disadvantages and afirst aspect of the invention is illustrated in FIG. 3. FIG. 3 is theanode portion 300 of a membrane electrode assembly fabricated inaccordance with the present invention. The anode portion 300 includesthe PCM 302. The PCM 302 may be constructed of a protonicallyconductive, electronically non-conductive membrane. One example of whichis NAFION®. NAFION®, as noted, is a cation exchange membrane based uponpolyperflourosulfonic acid, which is available in a variety ofthicknesses and equivalent weights.

A catalyst layer 304 is disposed on the NAFION® layer 302 using methodsknown to those skilled in the art, and the catalyst layer 304, as noted,may be a blend of platinum or ruthenium mixtures or alloys thereof. Thetop layer of the anode portion 300 of the MEA is an anode diffusionlayer 306. The anode diffusion layer evenly disperses the fuel, which isdirected towards the anode portion 300, as indicated by the arrow A,which fuel is reacted on the catalyst 304. The reaction producesprotons, which pass through the protonically-conductive, electronicallynon-conductive membrane 302.

The overall length of the NAFION® membrane 302 is indicated by referencecharacter 310. In accordance with the invention, the external borders ofthe NAFION® membrane 302 are rendered substantially impermeable toliquids by heating, ultrasonic treatments, chemically modifying thestructure of the NAFION®, or coating the membrane with an impermeablematerial, for example. Impermeable portions 312 and 314, visible in FIG.3 do not allow liquids, such as fuel or fuel and water, nor do theyallow protons carried by liquid water to pass therethrough. Instead, theliquid and thus the protons pass through the reactive portion 320. Weherein refer to this portion 320 of the NAFION® membrane 302 as the“permeable” portion of the membrane because it is permeable to liquids.

This can also be understood with reference to FIG. 4, which is aschematic top plan view of the anode portion 400 of the membraneelectrode assembly in accordance with the present invention. The anodeportion includes a NAFION® membrane 402, and a catalyzed layer 404,which as been either particle deposited or coated onto the NAFION®membrane 402. The catalyzed layer 404 is indicated by the dashed lines.The anode diffusion layer 406 (illustrated in the dot-dash lines) evenlydisperses the fuel across the relevant portion of the NAFION® membrane402.

In accordance with the invention, the NAFION® membrane 402 is renderedsubstantially liquid impermeable and protonically non-conductive in anouter peripheral area 410 (shown between the bold lines in FIG. 4). Thisouter peripheral area 410 is rendered substantially liquid impermeableby heat treating, ultra sonic treatment techniques, chemical treatmentsincluding amine dipping, and the like, or otherwise modifying theNAFION®. This area 410 may, but need not, be rendered substantiallyprotonically non-conductive by said heat treatment, or by using otherprocesses known to those skilled in the art, including but not limitedto amine dipping. This leaves the area 420 as the functionally activearea of the NAFION®, at which fuel that passes through the anodediffusion layer 406 is catalyzed at the catalyst area 404, to generateprotons that pass through the functional, permeable area 420 of theNAFION® membrane 402.

In addition to rendering the membrane border substantially liquidimpermeable, a physical deterrent is also provided by the presentinvention in that the relative dimensions of the components are selectedto aid in the prevention of leakage into undesired areas in a membraneelectrode assembly. As illustrated in FIG. 3, for example, the anodediffusion layer 306 is extended beyond the active or permeable area 320of the NAFION® membrane 302. This assists in preventing leakage becausefuel that diffuses through the portion of the diffusion layer 304, whichextends beyond the active area 320, will be catalyzed but will not passthrough the membrane. Thus, even though the fuel that is consumed bycatalysis in those areas does not contribute to electricity-generatingreactions, the fuel does not cross over the membrane and into thecathode area, which would otherwise lead to the negative consequenceswith respect to methanol cross-over, which have been discussed herein,such as wasting fuel and generating undesired heat on the cathode sideof the fuel cell.

FIGS. 3 and 4 illustrate one embodiment of the invention. It should beunderstood, however, that the physical portion of the membrane that isrendered substantially impermeable may be of various geometricconfigurations, and the invention is not limited to an active area thatis bordered on all sides by a rectangular impermeable area. There may beinstances in which multiple or other geometric sections of the membraneare be rendered liquid impermeable, depending upon the particularapplication of the invention, and these variations are contemplated asbeing within the scope of the present invention.

Another aspect of the invention is illustrated with respect to FIG. 5.FIG. 5 illustrates an anode portion 500 of a membrane electrode assemblythat is fabricated in accordance with the present invention. The anodeportion 500 includes a PCM 502, a catalyst layer 504 and an anodediffusion layer 506. Similar to that illustrated in FIG. 3, thecatalyzed portion 504 is larger than the anode diffusion layer 506 sothat fuel that is transported to the edges of, or around the edges ofthe anode diffusion layer 506 will still be consumed by catalysis. Inaccordance with this aspect of the invention, a fuel insulatingmaterial, such as TEFLON® is painted over the catalyst layer 504 ontothe membrane 502 in a border area as illustrated in cross-section inFIG. 5. This produces two portions, 512 and 514, which are sealedagainst leakage laterally around the diffusion layer 506. In addition,both the catalyst layer 504 and the diffusion layer 506 are larger thanthe permeable, active area 520 of the membrane 502. Thus, fuel thatdiffuses through the anode diffusion layer 506 is catalyzed on anunsealed portion 520 of the catalyzed membrane 502 contributed to theelectricity generating reactions.

A top plan view of this aspect of the invention is illustrated in FIG.6. FIG. 6 illustrates the anode portion 600 of a membrane electrodeassembly that is constructed in accordance with the present invention.The anode portion 600 includes a NAFION® membrane 602 that has acatalyzed area 604, indicated by the dashed lines and an anode diffusionlayer 606 that is indicated in the dot-dash lines. In this instance, aTEFLON® or similar sealant is painted in an area on the membrane suchthat overlaps the catalyzed layer and seals the MEA between thediffusion layer 606 and the catalyzed area 604 to physically preventleakage. This sealed area is generally designated by the referencecharacter 610 and is indicated by the area between the bold lines. Thesealed area 610 provides a physical barrier to leakage of fuel in andaround the anode diffusion layer, which allows an functionally activearea of NAFION® generally designated by the reference character 620.Even though in this embodiment the NAFION® membrane is chemicallycapable of conducting protons, the physical barrier still reduces wasteof fuel because the protonic conductivity is dominated by through-planetransport and there is very little lateral conductivity. Once again, theinvention is not limited to the embodiment shown in FIG. 6, insteadthere may be other portions of the membrane that are to be sealed, orthe sealed area may take a different shape than rectangular whileremaining within the scope of the present invention.

Nevertheless, the chemical sealing techniques of the present inventioncan be combined with the physical sealing techniques of the presentinvention as illustrated in FIG. 7. FIG. 7 is a schematic side sectionof an anode portion 700 of a membrane electrode assembly, which isfabricated in accordance with the present invention. The anode portion700 includes a PCM 702. The PCM 702 has a catalyzed layer 704 and ananode diffusion layer 706, associated therewith.

Similar to the embodiment of FIG. 3, a peripheral area of the PCMmembrane 702 has been treated, in part, such that it is liquidimpermeable. In the embodiment illustrated in FIG. 700, the liquidimpermeable portion is an outer border, i.e., areas 712 and 714resulting in a permeable area 720 that conducts protons in the mannerwhich allows for normal operation of the fuel cell.

The membrane 702 has a catalyzed layer 704. In accordance with theinvention, the length and/or width of the catalyzed layer 704 isselected so that it is longer than the permeable area 720 of the NAFION®membrane 702. This ensures that fuel leaking around the diffusion layeronto the outer borders of the diffusion layer is consumed by catalysisand does not cross over the membrane otherwise resulting in the negativeconsequences of methanol cross-over. A physical barrier is then placedover the borders of the catalyzed area 704. The physical barrier may beTEFLON®, which is impermeable to fuel, may be painted along thecatalyzed aspect along the outer portion of the catalyzed layer of themembrane, and illustrated as portions 716 and 718. As an alternative toTEFLON® an insulating coating, such as a gasketing, could also be usedfor this physical barrier. Thus, the diffusion layer 706, is selected tobe of a length smaller than the length of the catalyzed layer 704, tofurther resist leakage. Those skilled in the art will recognize thatthere are other means by which the peripheral areas of the NAFION® canbe modified such that it is impermeable to fuel. These include thosemethods listed herein or other methods not listed, used singly or incombination.

It is further possible to limit the flow of fuel by disposing a gasketor other component that physically impedes the passage of liquid fuelfrom the fuel source to the membrane electrolyte. Catalyst 804 isdisposed between anode diffusion layer 806 and membrane electrolyte 802(FIG. 8). Gaskets 808 a and 808 b are disposed in such a fashion thatfuel cannot pass from the fuel source to the membrane electrolyte, otherthan through the anode diffusion layer 806 and catalyst layer 804. Thegasketing is fabricated from silicone or other material which does notdegrade in the presence of methanol, and which is compatible with fuelcell construction, including but not limited to elastomers and plastics.As shown in FIG. 8, Gaskets 808 a and 808 b extend to the large aspectof the anode diffusion layer which is opposite the membrane electrolyte802, though a gasket which prevents the lateral transport of the fuelthrough the diffusion layer without covering a portion of large aspectof the anode diffusion layer is within the scope of the invention.

Those skilled in the art will recognize that these inventions may beimplemented on the cathode aspect of the membrane electrolyte in orderto achieve the goals of the invention.

These variations in constructing the membrane electrode assembly, inaddition to blocking fuel cross-over, allows for a margin of error inthe manufacturing process of the fuel cell as well. Other benefits to bederived from the invention include: reducing corrosion of other fuelcell components with which the fuel will have come in contact. Thiscould also eliminate cross-cell shunt current when a mono-polar cellconnection is used in the cell stack and achieves dimensional stabilityduring wet-dry cycles and under compression.

The foregoing description has been directed to specific embodiments ofthe invention. It will be apparent, however, that other variations andother modifications may be made to the described embodiments, with theattainment of some or all of the advantages of such. Therefore, it isthe object of the appended claims to cover all such variations andmodifications as come within the true spirit and scope of the invention.

1. A method of rendering a portion of a protonically-conductive membranesubstantially liquid-impermeable including the step of rendering aportion of said protonically-conductive membrane, substantiallyliquid-impermeable by heat treatment.
 2. The method as defined in claim1 including the further step of rendering said portion of saidprotonically-conductive membrane substantially liquid-impermeable byusing ultrasonic treatment techniques.
 3. The method as defined in claim1 including the further step of rendering said portion of saidprotonically-conductive membrane substantially liquid-impermeable byusing selected chemical techniques.
 4. The method as defined in claim 1wherein said selected chemical techniques include amine dipping.
 5. Themethod as defined in claim 1 including the further step of renderingperipheral edges of said protonically-conductive membrane substantiallyliquid-impermeable.
 6. The method as defined in claim 1 including thefurther step of rendering peripheral edges of saidprotonically-conductive membrane substantially impermeable by paintingan impermeable material along said peripheral edges.
 7. The method asdefined in claim 1 wherein said coating material includes TEFLON ®. 8.The method as defined in claim 1 including providing as said membrane, amembrane that is comprised substantially of NAFION®.
 9. The method asdefined in claim 1 including providing as said membrane, a membrane thatincludes a portion that has been rendered substantially protonicallynon-conductive.